THE SESSILE BARNACLES. 227 



Maxilla with a pair of larger spines near the lower angle. First 

 cirri with unequal rami; fourth cirri having recurved teeth on the 

 anterior margins of parts of the segments. 



Type. Balanus quadririttc/us Darwin. 



Distribution. Japan to the Great Barrier Reef of Australia. 



By its peculiarly armed fourth cirri, this group resembles some 

 species of Acasta, Mcmbranobalanus and Balanus; yet this character 

 is variable, and may merely indicate homoplastic modifications of 

 structures present in most groups of the genus. The opercular 

 valves, however, show great resemblance to those of Acasta and 

 Me nib ran obalanus, and I think indicate an actual affinity to both 

 groups, more especially the former. The shape of the tergum will 

 serve to separate the group from Sol'idolxdanus. 



Besides the species recorded below, the following, not contained 

 in the United States National Museum, belong to the group : 



Balanus terebratus Darwin. Key Islands. 



Balanus quinquevittatus Hoek. Latitude 1 42' 30" south; longi- 

 tude 130 47' 30" east. 



Balanus hystrix Hoek, placed by its author in this subgenus, has the 

 porous parietes and armed cirri of the amphitrite and tngonus 

 groups of Balanus. It differs from the present group also in the 

 form of the terga. See p. 78. 



KEY TO SPECIES OF ABMATOBALANUS. 



a 1 . Scutum with the growth-ridges crenulated, or showing longitudinal strife 

 in the intervals ; pit for the lateral depressor muscle well marked. 



6 1 . Radii broad, with level summits ; basis concave or flat, poreless ; 



scutum with the adductor ridge very indistinct or wanting. Ter- 

 gum moderately curved, its spur short, depending below the 

 basiscutal angle about one-frmrth of its own width B. allium. 



6 2 . Similar, but with the tergum deeply concave on the scutal mar- 



gin B. arcuatus. 



b 3 . Radii narrow ; parietes strongly ribbed ; tergum with the spur' longer, 

 depending below the basiscutal angle about half its own width. 



Scutum with a weak adductor ridge B. cepa. 



a 2 . Scutum with the growth-ridges smooth ; no longitudinal stria?. 



& 1 . Exterior smoothish ; basis thin, smooth, or not distinctly ribbed. 



c 1 . Four gray bands forming a cross, on rostrum, carina and lateral 

 compartments ; scutum without a distinct pit for the lateral 



depressor muscle B. quadrivittatus. 



c 2 . Five brownish spots, on rostrum and latera ; sis small apertures 



above the base in the sutures of the compartments. Scutum 



with a pit for the lateral depressor muscle_B. quinquevittatus. 



b. 1 Exterior white, strongly ribbed longitudinally, the ribs projecting 



at the periphery ; basis with corresponding projections, concave, 



radially ribbed, with little pits between the ribs. Tergum with a 



sharp chitinous beak B. terebratus. 



